What Shore A hardness is best for O-rings?

70 Shore A is the general-purpose default; 60–65 for low-temperature or high-compression service; 75–90 for high-pressure or abrasive duty where extrusion and wear resistance matter more than conformance.

Does harder rubber seal better?

Not always. Harder compounds resist extrusion and abrasion but need more compression force and seal poorly on rough or irregular surfaces, while softer compounds conform better to imperfect flanges.

How is Shore A tested?

Per ASTM D2240 / ISO 7619, a calibrated durometer presses a specified indentor into a flat specimen of adequate thickness and the reading is taken after about 15 seconds under a fixed load.

Shore A Hardness for Rubber & Silicone: A Buyer’s Guide

Q: What does Shore A measure?

Shore A is the durometer scale for flexible rubbers and elastomers, measuring indentation resistance from 0 (very soft) to 100 (rigid). Most molded rubber parts fall between 30 and 95 Shore A, with common seals around 40–90.

Author: Site Editor Publish Time: 2026-10-21 Origin: Site

When you specify a rubber or silicone part, one number drives almost everything downstream: Shore A hardness. It tells a buyer, engineer, and molder how stiff the part will feel, how much it will compress, and how it will behave under pressure and temperature. This guide explains what Shore A measures, the ranges used for common seals, and how to choose the right durometer for O-rings, gaskets, and custom molded parts.

What Does Shore A Measure?

Shore A is a durometer scale for flexible rubbers and elastomers. A calibrated gauge presses a spring-loaded indentor into the material; the depth of penetration is converted to a reading from 0 (very soft) to 100 (rigid). Softer gels and foams use the Shore 00 scale; plastics use Shore D. For molded rubber components, the useful window is roughly 30–95 Shore A, with most industrial seals falling between 40 and 90.

Why Hardness Matters for Sealing

Hardness is a trade-off, not a “higher is better” spec:

Soft (40–60 Shore A) – conforms to rough or irregular flanges, seals at low closure force, performs better at low temperature; more prone to extrusion and abrasion.
Medium (65–75 Shore A) – the default for general O-rings and gaskets; balances sealability with extrusion resistance.
Hard (80–95 Shore A) – resists extrusion, wear, and high pressure; needs more compression force and seals poorly on imperfect surfaces.

Typical Hardness by Part Type

The table below shows common durometer choices for standard rubber components. When an O-ring must also match a dimensional standard, pair the hardness with the correct size from the AS568 O-ring size chart.

Component	Common Shore A	Why
General-purpose O-rings	70	Balanced squeeze, compression set, and extrusion resistance
Low-temp / high-compression O-rings	60–65	Better cold flexibility and conformance
High-pressure static seals	75–90	Resists extrusion into the gap
Flat gaskets	50–70	Conforms to flange imperfections
Diaphragms	40–70	Flexibility and low stress relaxation
Vibration mounts / bumpers	45–75	Damping without bottoming out

How Shore A Is Tested

Hardness is measured per ASTM D2240 / ISO 7619. The durometer is placed on a flat specimen of adequate thickness (typically ≥6 mm) and the reading is taken after the indentor has rested for about 15 seconds under a fixed load. Readings on curved, thin, or uneven surfaces are not valid, so molded test buttons or finished-part flats are used for verification.

Choosing the Right Hardness

Start from the service condition, not a habit:

Pressure & gap – higher pressure and larger clearance call for harder compounds to avoid extrusion.
Surface finish – rough or warped flanges need softer rubber to seal.
Temperature – see our FKM temperature range guide for how heat shifts the effective hardness of each compound.
Media – chemical swell changes apparent hardness; check the material specification library for compatibility.

Custom Hardness for Molded Parts

Anlintech molds rubber and silicone parts across the full Shore A range in FKM, silicone, EPDM, NBR, HNBR, and FFKM. We verify durometer on every production lot and can hit tight tolerances per ISO 3302-1 for your drawing. Send the target hardness, service environment, and geometry for a quote; we provide material certification and hardness test reports with shipment.

Related Resources

AS568 O-Ring Size Chart — dash numbers with ID, OD, and cross-section in inches and mm.
Material Specification Library — temperature, hardness, and chemical-resistance data for every compound.
FKM (Viton) O-Rings — hardness and temperature options for fluoroelastomer seals.